1. Field of the Invention
This invention relates to a method of writing data to a semiconductor memory device with electrically rewritable and non-volatile memory cells.
2. Description of Related Art
A NAND-type flash memory is known as one of electrically rewritable (i.e., erasable and programmable) and non-volatile semiconductor memories (EEPROMs). The NAND-type flash memory has a feature that it is possible to make unit cell area thereof smaller than that of a NOR-type one, and thereby easily increasing the memory capacity, because it has a memory cell array with NAND cell units, each of which has a plurality of memory cells connected in series.
A transistor is used as a memory cell in such the flash memory device, which has a charge storage layer (for example, floating gate) and a control gate stacked thereabove. This memory cell usually stores binary data in such a manner that data “0” is defined as a high threshold voltage state (usually, positive threshold voltage state) where electrons have been injected into the floating gate; and data “1” is defined as a low threshold voltage state (usually, negative threshold voltage state) where electrons stored in the floating gate have been released.
To increase the data storage capacity of the flash memory, it may be used a multi-value data storage scheme so that one memory cell stores plural bits. For example, as a four-value data storage scheme, there has already been provided such a data bit assignment method that four-value data “xy”, which is defined by upper page data (i.e., upper bit data) “x” and lower page data (i.e., lower bit data) “y”, are arranged in order of threshold voltage as follows: “A”=“11”, “B”=“10”, “C”=“00” and “D”=“01” (refer to, for example, Unexamined Japanese Patent Application Publication No. 2001-93288).
According to this data assignment method, there is obtained advantageous effects such as: the number of lower page read operations may be decreased; and high-rate data write may be achieved.
There has been provided another data bit assignment example in which “A”=“11”, “B”=“10”, “C”=“01” and “D”=“00” are arranged in order of threshold voltage. In this method, however, it is necessary for reading data to do one upper page read operation and three lower page read operations.